Here is a CSS template for modifying input fields in CSS. With Sass,
you can achieve this by just changing a variable. The CSS shown below
is unprefixed. Depending on what you use, you may have to change the
type attribute selector.

Concerning SASS: It looks like there is only a global text color defined in Materialize, which will by default also apply to your input fields.
In _variables.scss you can find the $off-black variable which is responsible for the color. It's applied to <html> tag and changing it will therefore change the text color of your whole page.

Email codedump link for How to change the input field&#39;s text color in MaterializeCSS?